A user equipment uses a random access channel (RACH) to access a network in a state that the user equipment is not uplink synchronized with a base station. The user equipment can perform initial ranging and periodic ranging through the RACH. The user equipment performs the initial ranging to acquire downlink synchronization and first access a base station, and performs the periodic ranging to instantaneously access a network if necessary in a state that the user equipment is connected with the network. The initial ranging is used to allow the user equipment to acquire synchronization while accessing the network and receive desired user equipment (UE) identifier during communication. The periodic ranging for accessing the RACH is used to initiate a protocol to receive information from the base station or when a transmission packet exists.
The periodic ranging can be classified into two types depending on 3GPP LTE (Long Term Evolution), i.e., a synchronized access mode and a non-synchronized access mode. The synchronized access mode is used if an uplink signal is within a synchronization limit when the user equipment tries access through the RACH. The non-synchronized access mode is used if the uplink signal is beyond the synchronization limit. The non-synchronized access mode has the same concept as the periodic ranging, and is used when the user equipment accesses the RACH for the purpose of notifying the base station of the change status of the user equipment and requesting resource allocation. On the other hand, the synchronized access mode alleviates limitation of a guard time in the RACH by assuming that the user equipment does not depart from uplink synchronization with the base station. For this reason, much more time-frequency resources can be used. In the 3GPP LTE, a considerable amount of messages (more than 24 bits) may be added to a preamble sequence for random access in the synchronized access mode so that both the preamble sequence and the messages may be transmitted together.
Hereinafter, the RACH which is being currently discussed in the 3GPP LTE system will be described.
FIG. 1 illustrates a structure of the RACH according to the related art.
It is assumed that a channel structure of the RACH which is currently discussed has a bandwidth of minimum 1.25 MHz and a length of minimum 1 sub-frame.
Although FIG. 1 illustrates one sub-frame, the RACH of FIG. 1 may increase to reach N number of sub-frames on a time axis depending on a cell radius. A time-frequency resource (TFR) of FIG. 1 is a transmission unit in the LTE, and generation frequency of the RACH is determined depending on QoS requirements in MAC layer. In other words, the RACH is generated once in a unit of several tens of ms or several hundreds of ms. Signals transmitted to the RACH should be characterized in that their detection can easily be performed in a time domain. To this end, various methods have been suggested, which are generally based on CAZAC (constant amplitude zero auto-correlation) sequence. The CAZAC sequence can be classified into two types, i.e., GCL CAZAC sequence and Zadoff-Chu CAZAC sequence. First of all, the GCL CAZAC sequence is given by the following Equations 1 and 2.
                              c          ⁡                      (                          k              ;              N              ;              M                        )                          =                              b            ⁡                          (                              mod                ⁡                                  (                                      k                    ;                    m                                    )                                            )                                ·                      exp            (                          -                                                jπ                  ⁢                                                                          ⁢                                      Mk                    ⁡                                          (                                              k                        +                        1                                            )                                                                      N                                      )                                              [                  Equation          ⁢                                          ⁢          1                ]            (in case where N is an odd number)
                              c          ⁡                      (                          k              ;              N              ;              M                        )                          =                              b            ⁡                          (                              mod                ⁡                                  (                                      k                    ;                    m                                    )                                            )                                ·                      exp            (                          -                                                jπ                  ⁢                                                                          ⁢                                      Mk                    2                                                  N                                      )                                              [                  Equation          ⁢                                          ⁢          2                ]            (in case where N is an even number)
where b(mod(k;m) determines a length of a zero-correlation zone of GCL-CAZAC and usually uses Hadamard or complex exponential sequence.
The Zadoff-Chu CAZAC sequence is given by the following Equations 3 and 4.
                              c          ⁡                      (                                          k                ;                N                            ,              M                        )                          =                  exp          (                                    jπ              ⁢                                                          ⁢                              Mk                ⁡                                  (                                      k                    +                    1                                    )                                                      N                    )                                    [                  Equation          ⁢                                          ⁢          3                ]            (in case where N is an odd number)
                              c          ⁡                      (                                          k                ;                N                            ,              M                        )                          =                  exp          (                                    jπ              ⁢                                                          ⁢                              Mk                2                                      N                    )                                    [                  Equation          ⁢                                          ⁢          4                ]            (in case where N is an even number)
Examples of methods for transmitting data during random access using CAZAC sequence will be described below. The first method is to analyze CAZAC sequence ID as message information. The second method is to transmit CAZAC sequence and other sequence in a code division multiplexing mode, wherein CAZAC ID is used as unique UE identification information and other code mixed by the code division multiplexing mode is analyzed as message information. The third method is to mix the CAZAC sequence with another sequence (for example, Walsh sequence), wherein CAZAC ID is used as UE identification information and Walsh sequence is analyzed as message information. The fourth method is to directly perform data modulation for the CAZAC sequence, wherein CAZAC ID is used as UE identification information and modulated data is decoded to extract message. The fifth method is to transmit a message part attached to the CAZAC sequence, wherein the message part is transmitted in the same manner as the existing data transmission and CAZAC ID is used as UE identification information. The fifth method is mainly used in a synchronized random access channel.
Generally, the aforementioned data transmission methods can be classified into two types depending on message transmission through the RACH. In other words, the data transmission methods are classified depending on whether message is transmitted separately from a preamble sequence or transmitted by being implicitly included in the preamble sequence. In case of implicit transmission, the message occupies a time-frequency domain which is the same as that occupied by the preamble sequence. For example, in this case, CAZAC ID of the sequence is regarded as the message. If a sufficient number of messages that can be used as the preamble sequence are provided, message transmission can be performed with only sequence ID without additional manipulation. However, considering that maximum 24 bits are required when the RACH is actually implemented, it is difficult to obtain a sufficient number of sequence sets, and it takes the considerable cost required for detecting the sequence sets. For another example, sequence ID is simply used to identify a number used for random access by UE, and other additional information is transmitted simultaneously with preamble sequence.
FIG. 2 to FIG. 4 illustrate methods for transmitting a message a random access channel (RACH) in accordance with the related art.
Referring to FIG. 2, Walsh code which will be used as message is transmitted in a CDM mode simultaneously with the preamble sequence. According to the method of FIG. 2, the original CAZAC sequence is combined with Walsh sequence and then transmitted. In this case, since the original CAZAC sequence is combined with Walsh sequence, the method of FIG. 2 belongs to the CDM mode.
Referring to FIG. 3, Walsh sequence is directly mixed with the CAZAC sequence unlike the method of FIG. 2. In this case, message is identified as ID of Walsh sequence, and a base station uses the CAZAC sequence as UE identification information.
Referring to FIG. 4, another sequence is not mixed with the CAZAC sequence unlike the aforementioned methods. In case of the method of FIG. 4, data modulation is directly performed. In this case, data transmission can be performed for more data than those transmitted when the sequences are mixed. To transmit more data, messages may directly be transmitted following the preamble sequence as shown in (c) or (d) of FIG. 4.
The aforementioned transmission methods are to transmit much more message information while maintaining the characteristic of the CAZAC sequence if possible.
The aforementioned message transmission methods can be classified into two types in another aspect. That is, the one type is to use the sequence as it is while the other type is to attach another sequence to the original sequence. If the sequence is used as it is, the quantity of message which is transmitted increases to a value obtained by taking “log2” for the length of the sequence even though the length of the sequence becomes long. In this case, a problem occurs in that too long sequence is required for required message transmission. Meanwhile, if another sequence is attached to the original sequence, problems occur in that the characteristic of the CAZAC is degraded and performance may become bad rapidly depending on the channel status.